We will incubate suspensions of rat hepatocytes to investigate mechanisms controlling cholesteral synthesis, beta-hydroxy-beta- methylglutaryl CoA (HMG-CoA) reductase activity and the synthesis and secretion of all three classes of lipoproteins, (VLDL, HDL and LDL). Specifically, we will determine the effects of normal rat serum, epinephrine, insulin, corticosterone, glucagon and other hormones, lipoproteins, chylomicrons, and dispersions of lecithin and of lecithin- cholesterol on these parameters. We will continue the purification of solubilized HMG-CoA reductase by hydroxylapatite and DEAE-cellulose chromatography, prepare antibody to the pure homogeneous protein material obtained and use the antibody in differentiating between changes in reductase activity and amount of enzyme protein. This method will be used to study the mechanisms responsible for the circadian rhythm, for the rapid decrease in activity following cholesterol feeding, and for the rapid increases subsequent to epinephrine administration both in rats and in free hepatocyte suspensions. The reversible cold inactivation phenomenon of HMG-CoA reductase will be further investigated, and the possibility explored that the reductase may undergo reversible changes in activity due to changes in the lipid composition of the membrane. The mechanism of the rapid in vivo degradation of the reductase will also be explored.